British police say they have more than 20 suspects in the disappearance of Madeleine McCann, the British toddler who vanished from a family holiday in Portugal six years ago.

According to the Daily Mail, a review from Scotland Yard into how the case was handled by Portuguese authorities, identified dozens of fresh leads.

British police said work was under way to support police in Portugal, even though they had closed their investigation into the disappearance.

They new leads included "forensic opportunities" and several "people of interest", which included Britons, who had not been eliminated from the case.

The Mail reported Madeleine's parents, Kate and Gerry McCann were "greatly encouraged" by the review. Madeleine vanished from her family's vacation home in Portugal's Algarve region on May 3, 2007, days before her fourth birthday. She was snatched from the family's hotel room while her parents dined at a nearby restaurant.

The McCanns recently went back to the coastal town of Praia da Luz on the anniversary of Madeleine's disappearance.

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Officials in Lisbon have said they would not reopen the case unless new evidence was presented, while British police are confident the latest breakthrough could result in new and significant evidence.

But the Mail has reported a diplomatic row could be heating up, as Portuguese officials are refusing to reopen the case.

A source told the Mail it was like a "mexican stand-off".

"It's a chicken-and-egg situation. Significant new evidence can be found if the leads uncovered by the Yard are investigated.

"There are two major obstacles to a joint investigation: the money to fund it in Portugal and the loss of face they would suffer from having to agree to such an inquiry."

The case has generated intense media interest in Britain.

British police launched Operation Grange in 2011, to try to solve the puzzle.